Method for call establishment

ABSTRACT

The invention relates to methods for call establishment in a cellular radio system. In the inventive method, the visitor location register stores information (405) on whether the subscriber (CPP) is located within the service area of its home private branch exchange (PBX) indicated with a home location area identifier, in response to the location data request (106) placed by the mobile services switching center the visitor location register (VLR) checks the subscriber data of the subscriber and finds that the subscriber (CPP) is located within the service area of its home private branch exchange indicated with a home location area identifier, the visitor location register does not initiate the mobility management functions pertaining to normal connection establishment in the system.

FIELD OF THE INVENTION

The invention relates to a method for call establishment in a cellularradio system comprising a mobile services switching centre and a privatebranch exchange connected thereto, and a visitor location register forstoring subscriber and location data on subscribers located within theservice area of the mobile services switching centre, in which method:the mobile services switching centre is requested to establish a call toa subscriber, the mobile services switching centre requests the visitorlocation register to provide location data of the subscriber.

BACKGROUND OF THE INVENTION

Various cellular radio systems or mobile telephone systems are in useand being designed in which the geographic coverage area of the systemis divided into smaller separate service areas, i.e. cells, in such away that when the radiotelephone or mobile telephone is located in acell, it communicates with the fixed network through a fixed radiostation incorporated in the cell. The mobile telephones belonging to thesystem can roam freely within the area of the network from one cell toanother. One such system is the digital mobile telephone system GSM(Global System for Mobiles).

Cordless telephone (CT) systems, such as the DECT system, have recentlycome out on the market. The DECT system comprises a base station, whichis normally based on access to an existing fixed telephone network.There are three main types of base stations: a residential base station,a base station connected to a private branch exchange in an office, andtelepoint base stations.

Cordless telephone systems can also be connected to mobile telephonenetworks, for instance to the GSM system. The private branch exchange(PBX) of a cordless telephone system can be connected to the GSM systemfor example by using "DSS 1, Q.930-Q.940, 1989, Digital SubscriberSignalling System No. 1, Network Layer, User-Network Management, CCITT"signalling supplemented with mobility management messages. The PBX of acordless telephone system is connected to the switching centre of theGSM system such that the subscribers in the PBX can roam within theservice area of the GSM system, and the subscribers in the PBX can makeuse of the mobility management procedures of the GSM system and roamwithin the service area of DECT systems connected to the GSM system. Insuch a case, mobile subscribers are attached to the PBX in accordancewith the DECT radio path protocol "ETS 300175, Radio Equipment andSystems (RES), Digital European Cordless Telecommunications (DECT),Common Interface", for instance. It has also been possible to connectfixed subscribers to the PBX, for example in accordance with the ISDNsignalling protocol. In that event, also calls originating from the GSMsystem or from a GSM-connected telephone network can be routed tosubscribers, for instance DECT subscribers, located within the servicearea of the PBX.

In the prior art solution, in the case of an incoming call to the PBX,register interrogation procedures in accordance with the GSMrecommendations are performed, irrespective of the location of thecalled subscriber. FIG. 1a shows call establishment procedures inaccordance with the prior art. When the call arrives at a first mobileservices switching centre, which may be a gateway mobile servicesswitching centre (GMSC), the GMSC requests 101 for routing informationfrom the home location register (HLR) of the called subscriber bysending a SEND₋₋ ROUT₋₋ INFO message. The home location register HLRchecks the subscriber data from its subscriber database and sends 102 aroaming number request PROV₋₋ ROAM₋₋ NUM to the visitor locationregister VLR of the location area of the subscriber, and the VLR sends103 a roaming number and routing information as an acknowledgementROAM₋₋ NUM₋₋ ACK to the home location register HLR. The home locationregister HLR further sends 104 the routing information and roamingnumber as an acknowledgement ROUT₋₋ INFO₋₋ ACK to the gateway mobileservices switching centre (GMSC), which sends 105 information on thesubscriber, the incoming call and the roaming number in an initialaddress message (IAM) (specified in the common channel signallingstandard "Q.721-Q.766, 1989, Specifications of Signalling System No. 7,CCITT, Vol. VI--Fascicle VI.8, p. 251") to the mobile services switchingcentre MSC of the location area of the subscriber. The call may alsoarrive at the mobile services switching centre MSC directly from theMSC's own service area. In that case, the above gateway mobile servicesswitching centre GMSC is replaced with the same mobile servicesswitching centre MSC in the service area of which both the A subscriberand B subscriber are located. When the mobile services switching centreMSC has received information on the incoming call, it requests 106 thevisitor location register VLR of the location area of the subscriber toprovide routing information for the call to be routed by means of a S₋₋INFO₋₋ F₋₋ I/C₋₋ CALL message. After having received the message orrequest, the visitor location register VLR performs a subscriber datacheck and initiates the prior art mobility management functions whichform part of the normal operations for establishing a connection and areset forth in greater detail in the GSM recommendation "GSM 09.02, MobileApplication Part Specification, ETSI". The first mobility managementfunction is the paging function 107, by means of which the subscriber islocated prior to call establishment. After the paging, the mobileservices switching centre MSC sends 108 a resource request PROCESS₋₋ACC₋₋ REQ to the visitor location register VLR. This is followed byauthentication 109 of the subscriber. Authentication is a processwherewith the subscriber is identified as a legal user of the system.Subsequent to the authentication, ciphering 110 of the messages employedis performed. Thereafter the visitor location register VLR responds tothe resource request by sending 111 an acknowledgement ACCESS₋₋ REQ₋₋ACC. Following this, assignment 112 of the temporary mobile subscriberidentity TMSI₋₋ ASSIGNMENT of the mobile subscriber is performed. Theprocedure according to this Figure is continued in FIG. 1b.

FIG. 1b is a continuation of the prior art call establishment shown inFIG. 1. When all the connection establishment functions 106, 107, 108,109, 110, 111 and 112 shown in FIG. 1a have been performed, the visitorlocation register VLR sends 113 a COMPLETE₋₋ CALL message by which thevisitor location register indicates that a call can be established. Onlythen is the mobile services switching centre MSC free to establish acall to the subscriber. In that situation, the mobile services switchingcentre MSC sends a connection establishment request 114 as a SETUPmessage to the home private branch exchange PBX of the subscriber.Thereafter, the PBX sends a connection establishment CC₋₋ SETUP message119 to the subscriber station CPP, that is, to the hand-held mobiletelephone attached to the PBX. It is to be noted that fixed subscribersmay also access to the PBX. The subscriber station CPP responds to theCC₋₋ SETUP message by sending 120 a CC₋₋ ALERTING message to the PBX.Following this, the PBX sends 121 a corresponding ALERT message to themobile services switching centre MSC, which sends 122 an addresscomplete message (ACM) further to the gateway mobile services switchingcentre GMSC, indicating that all digits required for routing the callhave been received. When a connection has been established between thesubscriber station CPP and the PBX, the subscriber station CPP sends 123a CC₋₋ CONNECT message to the PBX, and the PBX transmits 124 thismessage to the mobile services switching centre MSC as a CONNECTmessage. The PBX acknowledges the message sent to the mobile servicesswitching centre by sending 125 a CC₋₋ CONNECT₋₋ ACK message to thesubscriber station CPP. Respectively, the mobile services switchingcentre acknowledges the received CONNECT message by sending 126 aCONNECT₋₋ ACK acknowledgement message to the home private branchexchange PBX of the subscriber. Thereafter, the mobile servicesswitching centre indicates that the subscriber has responded to the callby sending 127 a response message ANM to the gateway mobile servicesswitching centre GMSC.

It can be seen from the foregoing that establishment of a connection toa subscriber located within the service area of a private branchexchange connected to a cellular radio network is a complicated andcumbersome process. It should be noted that the prior art callestablishment procedure as set forth above must be performed always whena subscriber located within the service area of the private branchexchange is to be called. The mobility management functions to beperformed in the GSM system which form part of the operations forconnection establishment, i.e. the paging 107, subscriber authentication109, ciphering 110, TMSI assignment 112, the S₋₋ INFO₋₋ F₋₋ I/C₋₋ CALL106, PROCESS₋₋ ACC₋₋ REQ 108, ACCESS₋₋ REQ₋₋ ACC 111 and COMPLETE₋₋ CALL113 messages to be sent, the roaming number request PROV₋₋ ROAM₋₋ NUM102 placed by the home location register HLR and the response ROAM₋₋NUM₋₋ ACK 103 provided thereto by the visitor location register VLR, asdescribed in the GSM 09.02 recommendation, load considerably thesignalling capacity of the mobile telephone network used. Heavy loadingof the signalling capacity may lead to an overload and system failure,and naturally gives rise to useless traffic in the network.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a method for callestablishment avoiding the above problems when a call is routed to asubscriber either through the mobile services switching centre of thelocation area of the subscriber in the GSM system or through anothermobile services switching centre in the GSM system, to the home privatebranch exchange of the subscriber. The object is to diminish uselessmobility management functions pertaining to connection establishment aswell as sending of useless signalling messages.

This novel method for call establishment is achieved with the method ofthe invention, which is characterized in that the visitor locationregister stores information on whether the subscriber is located withinthe service area of its home private branch exchange indicated with ahome location area identifier, in response to the location data requestplaced by the mobile services switching centre the visitor locationregister checks the subscriber data of the subscriber and finds that thesubscriber is located within the service area of its home private branchexchange indicated with a home location area identifier, the visitorlocation register does not initiate the mobility management functionspertaining to normal connection establishment in the system performed bythe mobile services switching centre but sends to the mobile servicesswitching centre an indication of the fact that the call can beswitched, the mobile services switching centre sends a connectionestablishment request to the private branch exchange of the homelocation area of the subscriber, the home private branch exchange of thesubscriber performs the necessary operations for establishing aconnection and establishes a connection between the subscriber and themobile services switching centre.

Another embodiment of the invention relates to subscribers that arealways within the service area of their home private branch exchange.The second embodiment of the invention is a method for callestablishment in a cellular radio system comprising a mobile servicesswitching centre, a private branch exchange connected thereto and avisitor location register and a home location register for storingsubscriber and location data on subscribers, in which method the mobileservices switching centre requests, to establish a call to thesubscriber, the home location register of the subscriber to providesubscriber and location data of the subscriber on the basis of thesubscriber number. This method is characterized in that the homelocation register of the subscriber maintains a PBX parameter, if thesubscriber and location data of the subscriber are not found in the homelocation register on the basis of the subscriber number of thesubscriber, the home location register compares the subscriber number ofthe subscriber with said PBX parameter, and if the desiredcorrespondence between the subscriber number of the subscriber and thePBX parameter exists, the home location register forms a roaming numberfor the subscriber on the basis of the roaming parameter of the homePBX, said roaming number carrying information on the fact that thesubscriber is located within the area of its home PBX, and sends it tothe mobile services switching centre, one mobile services switchingcentre requests the visitor location register of the location area ofthe subscriber to provide routing information for the subscriber bysending a subscriber and location data request message pertaining to thesubscriber and containing said roaming number, if the subscriber data ofthe subscriber are not found in the visitor location register on thebasis of the roaming number, the visitor location register compares theroaming number with the subscriber home PBX roaming parameter that isstored by the visitor location register, and if the desiredcorrespondence between these exists, the visitor location register doesnot initiate the mobility management functions pertaining to normalconnection establishment in the system but sends to the mobile servicesswitching centre an indication of the fact that the call can beswitched, the mobile services switching centre sends a connectionestablishment request to the PBX of the home location area of thesubscriber, the home private branch exchange PBX of the subscriberperforms the necessary operations for establishing a connection andestablishes a connection between the subscriber and the mobile servicesswitching centre.

The invention is based on the idea that when there is an incoming callto a subscriber located within the service area of its home PBX,optimized signalling wherefrom useless signalling and procedural stepsare omitted is employed instead of normal GSM signalling. This isrealized by effecting the connection establishment initially incompliance with normal GSM call establishment, up to the point when themobile services switching centre of the location area of the subscriberrequests the visitor location register to provide information on thesubscriber and its location. In prior art connection establishment, thevisitor location register has at this point initiated the mobilitymanagement functions pertaining to connection establishment, i.e. thecomplex paging, authentication, ciphering and TMSI assignmentprocedures. In the present inventive solution, the visitor locationregister exceptionally sends an indication of the mobility managementfunctions performed back to the mobile services switching centre in theform of a COMPLETE₋₋ CALL message. By means of this message, the visitorlocation register indicates to the mobile services switching centre thatit can initiate normal establishment of a connection. In that situation,the mobile services switching centre sends directly a connectionestablishment request to the home PBX of the subscriber without anycumbersome mobility management functions pertaining to the connectionestablishment, which would normally have to be performed in any case.The connection establishment request is sent to the home PBX of thesubscriber, i.e. to the exchange in whose service area the calledsubscriber is located. Upon having received the connection establishmentrequest, the PBX performs the necessary mobility management functionslocally and establishes a connection between the subscriber and themobile telephone network.

The second embodiment of the invention is based on the fact that thehome location register identifies subscribers located within the servicearea of the home PBX of the subscriber on the basis of, for example, thesubscriber number of the subscriber, and the visitor location registeridentifies the subscribers located within the service area of the homePBX on the basis of, for example, a part of the roaming number. Thussubscriber or location data of the subscriber need not be stored in thehome location register or the visitor location register.

It is an advantage of such a call establishment method that it reducesinternal signalling traffic, i.e. internal messages, in the mobiletelephone system. Thus the load on the telecommunication connectionsbetween the network elements of the system is reduced and their risk ofoverload and likelihood of congestion is diminished.

A further advantage of the invention is the fact that it reduces theload on the network components of the mobile telephone network, such asthe subscriber registers, and diminishes the likelihood of congestionand risk of overload on the system.

Still another advantage of the invention is the fact that the inventionexpedites the connection establishment when a call is placed by asubscriber in a mobile telephone network, or another telecommunicationsnetwork having connection with a mobile telephone network, to asubscriber located within the service area of a PBX connected to themobile telephone network. Connection establishment is expedited, since"useless" mobility management functions are omitted from theestablishment of the connection.

It is an advantage of the second embodiment of the invention thatsubscriber and/or location data on the subscriber need not be stored inthe location registers of the system, i.e. in the home location registerand the visitor location register.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be set forth in greater detail withreference to the accompanying drawings, wherein.

FIGS. 1a and 1b are signalling diagrams of prior art call establishmentwith the necessary mobility management functions.

FIG. 2 is a signalling diagram of the call establishment procedure inaccordance with the invention.

FIG. 3 is a functions diagram of the updating of the home location areaidentity of a subscriber in different subscriber registers in a mobiletelephone network.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1a and 1b have been explained already in connection with thedescription of the prior art.

FIG. 2 is a signalling diagram of the call establishment procedure inaccordance with the invention.

If the A subscriber initiating a call is located within the service areaof another mobile services switching centre than the subscriber (Bsubscriber) or in a fixed telephone network, the mobile servicesswitching centre MSC receives information on the incoming call by way ofthe gateway MSC (GMSC) informing the subscriber of an incoming call. Inthis case, the mobile services switching centre receives information onthe incoming call and the subscriber as well as the necessary routinginformation, as is shown by means of reference numerals 101, 102, 103,104 and 105 in FIG. 1a.

On the other hand, if the A subscriber initiating the call is locatedwithin the service area of the same mobile services switching centre MSCas the subscriber (B subscriber), the mobile services switching centrereceives information on the call to be switched directly from the basestation controller or base station.

In the second embodiment of the invention, when a call arrives at afirst mobile services switching centre which may be a gateway mobileservices switching centre GMSC, the GMSC requests 101 the home locationregister HLR of the called subscriber to provide routing information bysending a SEND₋₋ ROUT₋₋ INFO message. The home location register of thesubscriber maintains a PBX parameter according to the invention. The PBXparameter may have been set for instance so as to correspond fully orpartly to the MSISDN (Mobile Subscriber ISDN) number of the subscriber.The home location register HLR checks the subscriber data from itssubscriber database, and since in this second embodiment of theinvention the subscriber data are not stored in the home locationregister or visitor location register, the home location registerperforms the following comparison operation: The home location registercompares the subscriber's MSISDN number, obtained in said SEND₋₋ ROUT₋₋INFO message 101 from the mobile services switching centre, with theinventive PBX parameter that it stores. If a part of the subscriberMSISDN number, for instance its prefix, corresponds to the PBXparameter, the home location register HLR knows that the subscriber is ahome subscriber in said PBX. In that situation, the home locationregister forms a roaming number for the subscriber on the basis of theroaming parameter of the home PBX that it stores in its database, saidroaming number carrying information on the fact that the subscriber islocated within the area of its home PBX. The formed roaming number maycomprise, for instance, wholly or in part said home PBX roamingparameter. Thereafter the home location register (HLR) sends 102 theroaming number it has formed as an acknowledgement, as a ROUT₋₋ INFO₋₋ACK message, to the gateway mobile services switching centre GMSC. It isto be noted that in this second embodiment of the invention, the homelocation register HLR does not send a roaming number request to thevisitor location register VLR. Having received the roaming number fromthe home location register HLR, the gateway mobile services switchingcentre GMSC sends 105 information on the subscriber, the incoming calland the roaming number to the mobile services switching centre MSC ofthe location area of the subscriber in an initial address message IAM,which is specified in the common channel signalling standard"Q.721-Q.766, 1989". The call may also arrive at the mobile servicesswitching centre directly from the mobile services switching centre'sown service area. In that case, the above-stated gateway mobile servicesswitching centre GMSC is replaced with the same mobile servicesswitching centre MSC in whose service area both the A subscriber and Bsubscriber are located. Having received information on the incomingcall, the mobile services switching centre requests 106 the visitorlocation register VLR of the location area of the subscriber to providerouting information for the call to be routed by means of a S₋₋ INFO₋₋F₋₋ I/C₋₋ CALL message. Having received the message or request, thevisitor location register VLR performs a check of the subscriber data.In the solution according to the second embodiment of the invention, thesubscriber data of the subscriber are not stored in the visitor locationregister VLR, and thus they cannot be found when they are sought in thedatabase of the visitor location register on the basis of the roamingnumber. In that situation, the visitor location register compares theroaming number obtained in the IAM message from the home locationregister through the mobile services switching centre with the home PBXroaming parameter that is stored in the visitor location register. Thehome PBX roaming parameter may be wholly or in part the same as theroaming number sent to the visitor location register. If the desiredcorrespondence exists between the roaming number and the roamingparameter, for instance such that the prefix of the roaming number isthe same as the PBX roaming parameter, the visitor location registerknows that the optimized signalling mode is to be employed. In such asituation, the visitor location register does not initiate the mobilitymanagement functions pertaining to normal connection establishment inthe system.

These mobility management functions are omitted also in the firstembodiment of the invention. Therein the visitor location registerstores information on whether the subscriber is located within theservice area of its home private branch exchange indicated with a homelocation area identifier. This information may be a flag stored in thedatabase of the visitor location register, for instance. When the mobileservices switching centre now places a location data request, thevisitor location register checks the subscriber data of the subscriberand finds that the subscriber is located within the service area of itshome private branch exchange indicated with a home location areaidentifier. In that situation, the visitor location register does notinitiate the mobility management functions pertaining to normalconnection establishment in the system but sends to the mobile servicesswitching centre an indication of this, of the incoming call and of thefact that optimized signalling is to be employed. Thus the mobiletelephone network does not perform the mobility management functionspertaining to normal connection establishment in the GSM system. Theseomitted "useless" signalling messages and mobile applications part (MAP)messages, each of which may comprise several messages and which togetherconstitute the mobility management functions, are denoted by referencenumerals 106, 107, 108, 109, 110, 111, 112 and 113 in connection withthe description of FIGS. 1a and 1b. Thus also the prior art "useless"register interrogation, paging and other functions performed by thenetwork elements participating in the sending and reception of thesemessages are omitted, and the operation of the system is made faster,simpler and lighter. In accordance with the invention, the mobileservices switching centre MSC of the location area of the subscribersends a connection establishment request 214 to the home PBX of thesubscriber. The connection establishment request corresponds to theSETUP connection establishment request indicated by reference numeral114 in FIG. 1b. A mobile services switching centre MSC operating inaccordance with the inventive method is thus capable of handling aCOMPLETE₋₋ CALL message 213 sent by the visitor location register VLR,that is, a connection establishment request arriving in such a new andunexpected situation, since the mobile services switching centre isprovided with a handling mechanism and suitable handling means, realizedfor instance in software, for such a message. As the next step, the homePBX of the subscriber starts performing the mobility managementfunctions pertaining to connection establishment in the service area ofthe PBX. The PBX may either perform all of the mobility managementfunctions set out hereinbelow, perform only part of them or omit them.The first function pertaining to mobility management is the pagingfunction 215, whereby the subscriber within the service area of the PBXis located. Thereafter the PBX activates the authentication 216 of thesubscriber within the service area of the PBX. Subsequent to this step,the PBX performs ciphering 217 of the messages employed on the radioroute. This is followed by assignment 218 of the temporary useridentification, TPUI₋₋ ASSIGNMENT (the temporary subscriber identityTPUI corresponds to the TMSI in the GSM). The TPUI may be the same as,or different from, the GSM TMSI. If the subscriber in the PBX is a DECTsubscriber, the mobility management functions, i.e. the paging,authentication, ciphering and TPUI assignment, are performed inaccordance with the DECT standard "European Telecommunication StandardETS 300 175-5: Radio Equipment and Systems. Digital European CordlessTelecommunications Common Interface, Part 5: Network layer". Thereafterthe call establishment is continued as normal prior art callestablishment in accordance with FIG. 1b in such a way that the messagesending functions denoted by reference numerals 119, 120, 121, 122, 123,124, 125, 126 and 127 and the consequent functions executed by thenetwork elements are performed in accordance with the prior art.

FIG. 3 is a functions diagram of the updating of the home location areaidentity of a subscriber in different subscriber registers in a mobiletelephone network. In implementing the method of the invention, onlythose subscribers in the PBX need to be indicated as subscribers of thehome PBX in the visitor location register VLR that also roam outside theservice area of the home PBX. This is necessary because if thesubscriber is always located within the service area of the home PBX,for example if it is a fixed subscriber, the default value canautomatically be set at said subscriber being located specificallywithin the service area of its own PBX. The subscriber is indicated as asubscriber of the home PBX by means of subscriber identity(IMSI=international mobile subscriber identity) and location areaidentity. In the invention, a home subscriber database 405 is formed inthe visitor location register VLR connected to the home PBX out of theidentifiers of those subscribers whose home location area is in a givenPBX located within the service area of said VLR. Thus one PBX forms onehome location area. Such a novel home subscriber database 405 containsthe identifier of said PBX, i.e. the identifier of the correspondinghome location area, and the IMSI of the subscriber concerned. The homesubscriber database 405 also stores a flag, for example one bit,indicating whether the subscriber is located within the area of its homePBX or not. The home subscriber database may be for instance as outlinedin the GSM recommendation "TS GSM 03.08, 1992, Organisation ofSubscriber Data, ETSI", particularly Table 1, page 18 therein, butadapted in such a manner that the inventive data are included therein.

As the subscriber CPP arrives in its home PBX and updates 401 itslocation by requesting location updating, sending a LOCATE₋₋ REQ messageto the PBX, the PBX sends 402 the location updating request further tothe mobile services switching centre MSC of the location area of thesubscriber. The mobile services switching centre MSC transmits 403 thelocation updating request further to the visitor location register VLRof the location area by sending an UPDATE₋₋ LOCATION message comprisingthe subscriber IMSI and the identifier of the home PBX of thesubscriber. The visitor location register checks 404 from the subscriberhome PBX specific home subscriber database 405 whether the identifier ofsaid subscriber is found in the home subscriber database. If theidentifier of the subscriber is found in the home subscriber database,the visitor location register updates in the home subscriber database ahome subscriber parameter, i.e. a flag indicating that the subscriber islocated within its home location area.

The drawings and the description pertaining thereto are only intended toillustrate the idea of the invention. The call establishment methodaccording to the invention may vary in its details within the scope ofthe claims. Even though the invention has been set forth in the abovemainly in the context of the GSM system, the invention may also be usedin other kinds of mobile telephone systems.

I claim:
 1. A method for call establishment in a cellular radio systemcomprising a mobile services switching centre and a private branchexchange connected thereto, and a visitor location register for storingsubscriber and location data on subscribers located within the servicearea of the mobile services switching centre in which methodthe mobileservices switching centre is requested to establish a call to asubscriber, the mobile services switching centre requests the visitorlocation register to provide location data of the subscriber, thevisitor location register stores information on whether the subscriberis located within the service area of its home private branch exchangeindicated with a home location area identifier, in response to thelocation data request placed by the mobile services switching centre thevisitor location register checks the subscriber data of the subscriberand finds that the subscriber is located within the service area of itshome private branch exchange indicated with a home location areaidentifier, the visitor location register does not initiate the mobilitymanagement functions pertaining to normal connection establishment inthe system performed by the mobile services switching centre but sendsto the mobile services switching centre an indication of the fact thatthe call can be switched, the mobile services switching centre sends aconnection establishment request to the private branch exchange of thehome location area of the subscriber, the home private branch exchangeof the subscriber performs operations for establishing a connection andestablishes a connection between the subscriber and the mobile servicesswitching centre.
 2. A method for call establishment as claimed in claim1, wherein in connection with the location updating of the subscribers,information is transmitted to the visitor location register on the factthat the subscriber is located within the service area of its home PBX.3. A method for call establishment as claimed in claim 1, wherein a homesubscriber database specific to each home private branch exchange isformed in the visitor location register connected to said mobileservices switching centre out of the identifiers of those subscriberswhose home location area said home private branch exchange is.
 4. Amethod for call establishment as claimed in claim 2, wherein a homesubscriber database specific to each home private branch exchange isformed in the visitor location register connected to said mobileservices switching centre out of the identifiers of those subscriberswhose home location area said home private branch exchange is.
 5. Amethod for call establishment as claimed in claim 3, wherein, as thesubscriber arrives in the service area of the home private branchexchange and performs location updating, the visitor location registerof the location area of the subscriber checks from the home PBX specifichome subscriber database whether the identifier of said subscriber isfound in the home subscriber database.
 6. A method for callestablishment as claimed in claim 1, wherein the identifier of the homelocation area of the subscriber is updated, if appropriate, in thevisitor location register of the home location area of the subscriber.7. A method for call establishment in a cellular radio system comprisinga mobile services switching centre, a private branch exchange connectedthereto, and a visitor location register and a home location registerfor storing subscriber and location data on subscribers, in whichmethodthe mobile services switching centre requests, to establish a callto the subscriber, the home location register of the subscriber toprovide subscriber and location data of the subscriber on the basis ofthe subscriber number, the home location register of the subscribermaintains a PBX parameter, if the subscriber and location data of thesubscriber are not found in the home location register on the basis ofthe subscriber number of the subscriber, the home location registercompares the subscriber number of the subscriber with said PBXparameter, and if the desired correspondence between the subscribernumber of the subscriber and the PBX parameter exists, the home locationregister forms a roaming number for the subscriber on the basis of theroaming parameter of the home PBX, said roaming number carryinginformation on the fact that the subscriber is located within the areaof its home PBX, and sends it to the mobile services switching centre,one mobile services switching centre requests the visitor locationregister of the location area of the subscriber to provide routinginformation for the subscriber by sending a subscriber and location datarequest message pertaining to the subscriber and containing said roamingnumber, if the subscriber data of the subscriber are not found in thevisitor location register on the basis of the roaming number, thevisitor location register compares the roaming number with thesubscriber home PBX roaming parameter that is stored by the visitorlocation register, and if the desired correspondence between theseexists, the visitor location register does not initiate the mobilitymanagement functions pertaining to normal connection establishment inthe system but sends to the mobile services switching centre anindication of the fact that the call can be switched, the mobileservices switching centre sends a connection establishment request tothe private branch exchange of the home location area of the subscriber,the home private branch exchange of the subscriber performs operationsfor establishing a connection and establishes a connection between thesubscriber and the mobile services switching centre.
 8. A method forcall establishment as claimed in claim 7, wherein the correspondencebetween the subscriber number of the subscriber and the PBX parameter issuch that part of the subscriber number of the subscriber corresponds tothe PBX parameter.
 9. A method for call establishment as claimed inclaim 7, wherein the desired correspondence between the roaming numberand the subscriber home PBX roaming parameter is such that part of theroaming number corresponds to the roaming parameter of the home PBX. 10.A method for call establishment as claimed in claim 8, wherein thedesired correspondence between the roaming number and the subscriberhome PBX roaming parameter is such that part of the roaming numbercorresponds to the roaming parameter of the home PBX.